1. Field of the Invention
The present invention relates to a drive circuit for an ultrasonic motor which employs ultrasonic vibrations.
2. Related Background Art
Ultrasonic motors which employ ultrasonic vibrations are nowadays becoming widely used in many fields, due to their possession of various desirable characteristics such as high torque, capability for being made compact, low electrical consumption, etc.
FIG. 1 is a perspective view of an ultrasonic motor. Referring to this figure, the reference numeral 1 denotes an elastic body, on the lower surface in the figure of which there is fixed a piezoelectric element 2. As shown in FIG. 2, the piezoelectric element 2 is formed with a plurality of segments which are arranged around it in circumferential sequence with a pitch equal to half the wavelength .lambda. of a traveling wave which is to be set up, and the polarity of neighboring segments is opposite. In FIG. 2, the segments labeled with the symbol "X" (for example the segment 2a) have a polarity perpendicular to the plane of the drawing paper in the direction away from the viewer, and the segments labeled with the symbol "." (for example the segment 2b) have the opposite polarity (towards the viewer).
Further, as shown in FIG. 3, an electrode 2d and an electrode 2f are provided on the under surface of the piezoelectric element 2. Each of the electrodes 2d and 2f are formed by painting an electroconductive paint over eleven neighboring ones of the segments of the piezoelectric element 2. A first high frequency AC voltage is supplied to the electrode 2d via a reed wire 2e, and a second high frequency AC voltage is supplied to the electrode 2f via a reed wire 2g. In the following description, the groups of segments of the piezoelectric element 2 to which the first and the second high frequency AC voltages are supplied will respectively be collectively termed the A group and the B group as shown in FIG. 3.
When high frequency AC voltage is supplied to each of the electrodes 2d and 2f of the piezoelectric element 2 shown in FIG. 3, the piezoelectric element 2 is excited to vibrate, and generates traveling waves in the surface of the elastic body 1 which is grounded as shown in FIG. 1. The elastic body 1 and the piezoelectric element 2 together constitute a stator assembly ST.
Referring to FIG. 1, the reference numeral 3 denotes a rotor member, to the lower surface of which in FIG. 3 there is fixed or bonded a slider member 4 which is made of resin or the like. Since this rotor member 3 is kept pressed against the stator assembly ST by a pressure application means not shown in the figures, the rotor member 3 and the slider member 4 are rotated together when traveling waves are generated in the elastic body 1 as described above. The rotor member 3 and the slider member 4 together constitute a rotor assembly RO.
FIGS. 4A through 4D are figures for explanation of the theory of how the traveling waves are generated in the elastic body 1. Although as shown in FIG. 2 the piezoelectric element 2 is actually formed in a ring shape, in FIG. 4A the piezoelectric element 2 is shown as extended along a straight line for the convenience of explanation. FIGS. 4B through 4D show the basics of the generation of transverse vibrations in the elastic body 1 when mutually differing high frequency AC voltages are supplied to the A group of electrodes and to the B group of electrodes.
The horizontal axes of the waveforms displayed in FIGS. 4B through 4D show segment position, i.e. angular position along the piezoelectric element 2 around the central axis of the ultrasonic motor, while the vertical axes show the magnitude of the transverse displacement of the corresponding point on the elastic body 1, i.e. its vibrational amplitude. Further, the left portions of FIGS. 4B through 4D symbolically show (in vector form) the two sine wave voltage waveforms V1 and V2 which are being supplied to the A group of electrodes and to the B group of electrodes respectively, and the projections onto the horizontal axis of the conceptual rotating voltage vectors V1 and V2 show the actual voltage values at various time points of these voltages, with positive electrical potential being shown by a projection which extends rightwards and negative electrical potential being shown by a projection which extends leftwards. In FIGS. 4A through 4D, it is shown that the phase difference in time between the first high frequency AC voltage V1 which is being applied to the A group of electrodes and the second high frequency AC voltage V2 which is being applied to the B group of electrodes is maintained at .rho./2. Further, since as shown in FIG. 2 there is provided an empty segment in the space between the A group of electrodes and the B group of electrodes having width (circumferential extent) .lambda./4, therefore there is a phase difference in space between V1 and V2 of .lambda./4.
FIG. 4B shows the waveform of the transverse displacement of the elastic body 1 when the first high frequency AC voltage V1 is being applied to the A group of electrodes while no voltage is being supplied to the B group of electrodes; while FIG. 4D shows the opposite case, i.e. shows the waveform of the transverse displacement of the elastic body 1 when the second high frequency AC voltage V2 is being applied to the B group of electrodes while no voltage is being supplied to the A group of electrodes. If high frequency AC voltage were supplied either only to the A group of electrodes or only to the B group of electrodes, then the corresponding waveform shown in FIG. 4B or FIG. 4D would be a standing wave, and no traveling waves would be generated in the elastic body 1.
However, FIG. 4C shows the waveform of the transverse displacement of the elastic body 1 when the first and the second high frequency AC voltages V1 and V2 are being applied to the A and B groups of electrodes, respectively. In this figure, the hypothetical waveform Z1 shown by a dashed line represents the waveform of the transverse standing wave displacement of the elastic body 1 which would be generated by the application of only the first high frequency AC voltage V1 at its current amplitude to only the A group of electrodes, i.e. is a reduced version of the FIG. 4B waveform; and similarly the hypothetical waveform Z2 also shown by a dashed line represents the waveform of the transverse standing wave displacement of the elastic body 1 which would be generated by the application of only the second high frequency AC voltage V2 at its current amplitude to only the B group of electrodes, i.e. is a similarly reduced version of the FIG. 4D waveform. And the combination of these two waveforms Z1 and Z2 results in the combined waveform Z shown by the solid line, which is the waveform of a traveling wave which is progressing along the elastic body 1 in the rightwards direction in the figure. This traveling wave in the elastic body 1 thus is generated by the application of the high frequency AC voltages V1 and V2 to the A group of electrodes and to the B group of electrodes respectively, these voltages V1 and V2 having a phase difference in time of .rho./2and a phase difference in space of .lambda./4. The rotor assembly RO is rotated by this traveling wave, since characteristic resonant vibrations are also set up in the stator assembly ST. In the prior art control was performed so as to ensure that the frequency of the high frequency AC voltage which was being applied to the piezoelectric element 2 agreed with the resonant frequency of the stator assembly ST.
However, since the resonant frequencies can change according to change in the load on the motor and the like, drive circuits for ultrasonic motors have been proposed which can drive the motor at high efficiency and stably even when the resonant frequencies change.
For example, with a drive circuit of the type disclosed in Japanese Patent Laid-Open Publication No. 62-203575, the frequency of the high frequency AC voltage applied to the piezoelectric element 2 is changed according to the value of a monitor voltage generated by a monitor electrode 2c provided on the piezoelectric element 2, and thereby the rotor assembly is rotated steadily and reliably.
FIG. 5 is a block diagram of this type of prior art drive circuit for an ultrasonic motor. Referring to this figure, the reference numeral 11 denotes a high frequency signal generator which generates the high frequency AC voltage which is applied to the piezoelectric element 2, and 12 is an amplifier which amplifies this high frequency AC voltage from the high frequency signal generator 11, 13 is a phase shifter which shifts the phase of the high frequency AC voltage generated by the high frequency signal generator 11 by exactly .rho./2, and 14 is another amplifier which amplifies the high frequency AC voltage from the phase shifter 13. The high frequency AC voltages amplified by the amplifiers 12 and 14 are respectively fed to the electrodes 2d and 2f of the piezoelectric element 2.
The reference numeral 15 denotes a voltage detection section which detects the monitor voltage which is generated by the electrode 2c of the piezoelectric element 2. Since this monitor voltage changes according to the amplitude of vibration of the stator assembly ST, the voltage detection section 15 can measure the value of the amplitude of vibration of the stator assembly ST by detecting the value of this monitor voltage.
The reference numeral 16 denotes a voltage setting section for setting the value of the voltage as required for rotating the rotor assembly RO at the desired rotational speed, while 17 is a comparison section which compares the monitor voltage detected by the voltage detection section 15 with the reference voltage set by the voltage setting section 16. 18 denotes a frequency determination section which selects a desired frequency according to the comparison result produced by the comparison section 17. When it is decided by the comparison section 17 that the monitor voltage is lower than the reference voltage, then the frequency determination section 18 selects a relatively low frequency; while, when the monitor voltage is higher than the reference voltage, then the frequency determination section 18 selects a relatively higher frequency. The frequency selected by the frequency determination section 18 is input to the high frequency signal generator 11, whereby a high frequency AC voltage of this frequency is applied to the electrodes 2d and 2f of the piezoelectric element 2. By performing this sort of feedback control, it is possible to keep the rotor assembly RO rotating at the desired rotational speed in a stable manner.
However, with this type of drive circuit for an ultrasonic motor, when the high frequency AC voltage is applied to the piezoelectric element 2, the resonant frequency and vibrational amplitude inevitably deviate from their expected values, due to the inevitable influences of the manufacturing tolerances for the dimensions of the elastic body 1, for the dimensions of the various segments of the piezoelectric element 2, for the polarities of the various segments of the piezoelectric element 2, and for the thickness of the adhesive layer between the elastic body 1 and the piezoelectric element 2; in the following discussion, all these inaccuracies will be referred to collectively as manufacturing errors of the stator assembly ST.
FIG. 6 is a graph showing along the horizontal axis the frequency of the high frequency AC voltage which is being applied to the piezoelectric element 2, and along the vertical axis the amplitude of vibration of the stator assembly ST. The resonant vibration curve 21 in the figure is for the case when high frequency AC voltage is applied only to the A group of electrodes of the piezoelectric element 2, while the resonant vibration curve 22 is for the case when high frequency AC voltage is applied only to the B group of electrodes; and the resonant frequencies for these curves are designated as fA and fB respectively. When manufacturing errors occur in the production of the stator assembly ST, even if the frequencies and the amplitudes of the high frequency AC voltages applied to the A group of electrodes and to the B group of electrodes are the same, as show in the figure, deviation will occur in the resonant frequencies or amplitudes of the stator assembly ST. Accordingly, deviation occurs in the amplitude of the standing waves due to the A group of electrodes and in the amplitude of the standing waves due to the B group of electrodes, and, when these standing waves are combined in the elastic body 1, the resultant wave not only consists of the desired traveling wave component, but also includes a residual standing wave component which is very undesirable. Not only does this standing wave component not contribute in any manner to the rotation of the rotor assembly RO, but it is also actually a primary cause of deterioration in the efficiency of rotation of the rotor assembly RO. However, with a prior art type of drive circuit for an ultrasonic motor like the one shown in FIG. 5, the method of control does not take any account of the generation of this undesirable standing wave component.